Any institution calling itself the world's greatest deliberative body cannot be short on self-esteem. Nor is the United States Senate shy in asserting its prerogatives. Self-inflation and self-assertion are natural, indeed useful, institutional qualities in a system of separated powers -- except when they cause direct injury to the country as a whole.

Such is the case in two areas where the Senate is now asserting its authority against the president. In both the Senate is unpardonably putting institutional self-interest above the national interest. Worse, at the center of both is perhaps the ablest and most thoughtful Democrat in the Senate, Sam Nunn.

Nunn is mightily agitated by the administration's attempt to reinterpret the Anti-Ballistic Missile (ABM) Treaty to allow the testing and development of exotic Star Wars technology. Nunn believes, with some justice, that what the Senate was given to ratify in 1972 was a narrower interpretation of the treaty prohibiting such testing. That, argues Nunn, should now be the operative definition. Otherwise, if the administration can present a treaty under one interpretation and then after Senate ratification act under another interpretation, the Senate role of adviser and consenter is demeaned.

The State Department's legal adviser, Abraham Sofaer, argues that the full record is somewhat ambiguous, but that the weight of the evidence supports the administration's broader interpretation. His is not the majority view, but it is not eccentric: many who were involved at the time and since -- Henry Kissinger, Paul Nitze (who helped negotiate the treaty), and Sen. Ernest Hollings (who participated in the 1972 ratification debate) -- think the broad interpretation more correct.

Believe whom you will. It happens that at this moment the United States and the Soviet Union are in an advanced stage of the delicate negotiations on cutting strategic nuclear arsenals. And one of the Soviet demands is American acceptance of a narrow interpretation of the ABM Treaty.

Some administration officials, including Nitze, want to negotiate an ABM Treaty interpretation in exchange for radical cuts of offensive nuclear arms. But Nunn wants to give it away for nothing. He and his colleagues in the Senate have amended the defense authorization bill to deny money for any SDI testing beyond the narrow interpretation.

To legislate the Soviet position of a delicate arms control issue, in the midst of negotiations, is simply irresponsible. Whether or not the Soviet position is legally correct is not just unclear, it is irrelevant. If the negotiations fail, the Senate will have ample time to impose its view of the ABM Treaty on the president and thus preserve its own prerogatives as a coequal treaty maker. But to undercut the administration's position in Geneva -- for better or for worse, it is the United States position -- is to miss the nation's interest for the Senate's.

Senate jealousy of its prerogatives extends not just to treaty-making but to war-making as well. Since the U.S. Navy shot up a mine-laying Iranian ship, the clamor in Congress to invoke the War Powers Act grows daily. However, recognizing how flawed is the original War Powers Act, Senate leaders, including Nunn and Byrd, have been trying to push through a one-time imitation that would terminate the American reflagging of Kuwaiti vessels within 90 days unless both houses of Congress expressly vote otherwise.

It is important to note that the war powers struggle is not a partisan one. No president of either party has ever invoked or even accepted the War Powers Act. That is because it is an abominable piece of legislation. It sets a 60-90 day deadline for American withdrawal from any hostile environment unless Congress votes otherwise. As we go in anywhere, it announces that we already have one foot out. It thus invites any enemy wishing to ensure our leaving to hurry up and kill as many American soldiers as possible within the deadline period.

The Byrd, Nunn et al. substitute is not much better. It is a public declaration of American irresolution, and it gives the Iranians 90 days within which to concentrate their attacks to exploit that irresolution.

Nunn and Byrd are by no means calling for a U.S. pullout from the Gulf. The point is again institutional. They want Congress be a partner in the shaping of Gulf policy. But if Congress really wants to be involved, there is a far better way to do so. Vote up or down today: either cut off funds for the president's policy or support it and then let him manage it in quiet consultation with congressional leaders. Better in or out than maybe. Because maybe declares to our enemies: we're in, unless you can bloody us enough to panic Congress and drive us out.

The Senate has every right to assert its prerogatives, but it should think of the country too. Of course Congress has a role to play in concluding treaties and deploying troops in danger zones. But taking the Soviet position on a delicate negotiating issue and broadcasting American timidity in the Gulf by setting deadlines for would-be Islamic martyrs to shoot for is no way to exercise that role.

Congress is asserting itself against a president gravely weakened by the loss of the Senate and the Iran-contra scandal. Politics abhors a vacuum, and with Reagan turning into one, it is not surprising that Congress should seize an opportunity to occupy a bit of presidential space. Every institution is jealous of its power. But the U.S. Senate should be equally jealous of the nation's interest. On ABM and the Gulf, however, the Senate's gaze rests fixedly on itself.